1. Field of the Invention
The present application relates to an over-current protection device, and more particularly to an over-current protection device having low resistivity, superior resistance repeatability and good cycle life test results.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Because the resistance of a conductive composite material having positive temperature coefficient (PTC) characteristic is very sensitive to temperature variation, it can be used as the material for current sensing devices, and has been widely applied to over-current protection devices or circuit devices. The resistance of the PTC conductive composite material remains extremely low at a normal temperature, so that the circuit or cell can operate normally. However, when an over-current or an over-temperature event occurs in the circuit or cell, the resistance instantaneously increases to a high resistance state, e.g., at least 102Ω, so as to suppress over-current and protect the cell or the circuit device.
In general, the PTC conductive composite material contains crystalline polymer and conductive filler. The conductive filler is dispersed uniformly in the crystalline polymer. The crystalline polymer is usually a polyolefin polymer such as polyethylene. The conductive filler usually contains carbon black powder. However, carbon black exhibits low electrical conductivity and therefore is unsatisfactory to the demands of low resistivity applications.
U.S. Pat. No. 8,446,245 discloses the use of mixture of conductive ceramic filler and carbon black in an over-current protection device to acquire good performance on volumetric resistivity and resistance repeatability. The resistance jump R100/Ri of the over-current protection device is about 3-20, where Ri is initial resistance and R100 is the resistance after tripping 100 times. With the advancement of the over-current protection device, stricter requirement to life cycle is demanded to avoid the device being burned out in long-term usage. In the present application, an improved over-current protection device to U.S. Pat. No. 8,446,245 is devised in obtaining better performance on life cycle tests.